Thanks DrIvanKO! That did the trick for me.

I'm having some issues with JavaScript random tests that appear to have BigInt inputs and expect BigInt results because returning a Number yields:
lastDigit(5143600, 0: expected 1 to equal {}

If I convert the results to BigInt when the inputs are BigInt, this error is thrown:
TypeError: Do not know how to serialize a BigInt at JSON.stringify

Seems to be related to this issue with JSON.stringify and BigInt https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/BigInt_not_serializable, so maybe a test issue rather than a problem with my solution?